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Troll A Platform: The Heaviest Structure Ever Moved

70 kilometres (43 miles) off the west coast of Norway lies a beast of gargantuan size. A monster so large it remains the tallest and heaviest object ever moved across the surface of the Earth. To see it today you get but a glimpse of its true magnitude, the rest sits deep in the icy waters of the North Sea.

Troll A may no longer be the largest offshore platform in the world (that accolade now belongs to the BERKUT PLATFORM off the coast of Russia) but it is still unspeakably big. Measuring 472 metres (1,549 ft) it is taller than the PETRONAS TOWERS in Kuala Lumpur – and weighing a colossal 1.2 million tons, which is one and half times the weight of the Golden Gate Bridge in San Francisco – the statistics surrounding Troll A are simply dizzying – and remember, it was moved.  


Troll A Platform
Troll A Platform by Swinsto101 is licensed under CC-BY-SA

The Troll A natural gas platform lies in the Troll gas fields off the coast of Norway, an area where gas was discovered in 1979 and production began in 1995. The gas which lies roughly 1,400 metres below the sea is expected to be extracted for the next 70 years at current rates.  

There are currently three platforms in operation, Troll A, Troll B and Troll C. You might expect that Troll A was the first, but in fact, it was Troll B which began operations in 1995 – though Troll A, as a construction, was started before. 

As I just mentioned, what we see today doesn’t tell the whole picture, because the large gas platform visible is but the peak of this behemoth. Four titanic concrete supports stretch 303 metres (995ft) below the surface. Standing on the helicopter pad on deck is the same as standing at the top of the EMPIRE STATE BUILDING – if the sea reached up to the 80th floor that is.

Building something of this scale to sit in the hostile North Sea is a perilous endeavour, and Norway has experience of what can happen when it all goes catastrophically wrong. On 27th March 1980, the ALEXANDER L. KIELLAND, a semi-submersible drilling rig located 320 km (198 miles) east of Dundee in Scotland, collapsed, killing 123 of the 212 people on board. The cause was found to be a fatigue crack located in the bracing that connected the legs. These are dangerous places to work, between 2003 and 2013 an average of 108 workers died on oil or gas rigs around the world. 


Now to say that this construction was complex would be a glorious understatement. When we see it today it’s difficult to imagine that it was built on land, then towed by boat out to sea. The process was divided into two with the main gas platform providing the relatively straightforward build, and the four concrete legs the vastly more difficult.  

Construction of the Troll A platform began in 1991 and was finished in 1996. The giant legs were constructed using a continuous pour method known as slip forming. This is a process that was first pioneered in Minnesota in 1899 to build concrete grain silos and is now generally regarded as the fastest way to build a continuous concrete structure. The concrete power station towers that have now become common sight are typically built in this way.  

Oil Platform, building process
Oil Platform, building process by Bluemoose is licensed under CC-BY-SA

This method calls for concrete to be poured continuously to provide a seamless, water-tight structure – and when I say continuous I really mean it. To provide the ideal structure, concrete needs to be poured 24 hours a day. As the concrete set below, cranes winched the halo, a platform from which the concrete could be poured from, sitting above the legs higher and higher. But this was not a quick job and took 20 minutes to pour just 5cm of concrete, in total it took 100 days to pour the concrete for just one of the towers.  

But normal concrete would not do on the Troll A. The North Sea is a hostile environment and Troll A is regularly hit by waves of up to 30 metres high. Over a period of 70 years engineers estimated that the Troll A would be hit by roughly 180 million of these large waves. The support towers needed to be incredibly strong, but also flexible to be able to bend slightly under pressure.  

They were made using flexible concrete, a concept first introduced by a gardener at the Palace of Versailles in the 19th Century. The French gardener first introduced iron to his concrete, but today we use steel. Essentially steel grids are placed at different levels and the concrete is poured onto it. This gives the concrete a significantly higher degree of flexibility. 

On Troll A they used enough steel to construct 15 Effiel Towers and enough concrete to build two and a half Wembley Stadiums. Each leg is 365 metres (1200ft) tall and measures 2 metres in thickness. They rise in perfect unison, thicker at the bottom, thinner in the middle and larger again at the top, giving the underwater structure a conical shape. These legs have been built to keep the Troll A Platform standing until at least 2066. 

One of these legs contains an elevator which can be used by arriving boats but also travels down to the seabed, a journey that takes 9 minutes. It is from here that engineers can work on the various pumps working to bring gas up from the sea bed.  

Near the bottom of the legs is a fortified boxing which connects all of the four structures, while a brace is located about halfway up which again connects each of the legs This means that the wave action is disrupted more equally among the legs. Interestingly, designers were not so worried about the larger sporadic waves, but the smaller waves constantly hitting the same area with the same frequency. The fortified boxing and brace add the kind of support that can alleviate this problem. 

At the very base of the legs are six 40 metres (130 ft) tall vacuum-anchors which hold the entire structure firmly in place on the sea bed. This gravity-based technology use anchors spread widely, a little like a footprint, that then have the air sucked out of them The suction created is essentially the same as when you fill a glass of water then place it upside on a surface – but on a much larger scale of course.   


The most perilous part of the entire process came when the platform and the legs needed to be joined. I guess they had two options here, raise the platform to the top of the 365 metres (1200ft) towers, or lower the legs – they chose the second. Once again, Norway had an experience of what can go terribly wrong in this situation. Just five years earlier the SLEIPNER A was in the middle of exactly the same operation. On 23rd August 1991, the hull of the Sleipner A was in midst of being lowered into the water near Gandsfjord. 

As it reached the 65-meter (213 ft) mark, a low rumbling noise could be heard and suddenly the hull was falling at 1 metre every minute, much faster than the planned 1 metre every 20 minutes. Falling further and further into the fjord, the buoyancy tanks imploded shortly before it hit the floor, which caused an earthquake measuring 3 on the Richter scale. The cause was concrete walls that had been designed too thin to withstand the pressure as it dropped. 

Thankfully nobody was hurt during the accident, but lessons had to be learned. There would be no such problems on the Troll A. The giant concrete legs were slowly submerged and filled with water, almost to the top, then the platform was manoeuvred into place just a few metres above the legs. 

Next, water was pumped out of the legs which began to slowly rise centimetre by centimetre until the four legs and the platform joined. The two were then securely attached, and the two sections of Troll A had become one. 

To make things just that little bit more complicated, the entire structure needed to be raised so it could clear the reef that lay between the assembly point and the open sea. Again, this was done by pumping out the water within the legs until it reached the desired height.  


The journey of Troll A from its fjord base to its final location proved to be an enormous television draw in Norway. A total of ten boats were used to shift the mighty structure 200 kilometres (120 miles) from the small village of Vats where it had been built, to the Troll gas fields, a trip that took them seven days. 

Once it arrived at its location it was lowered to the seafloor, its vast bulk pressing down into the soft sludge at the bottom. The taps on the vacuum anchors were then closed, creating a suction powerful enough to hold the vast bulk in place. Troll A was finally ready.     


Gas is pumped from 40 wells located at the base of the four legs and is exported directly to a processing plant in KOLLSNES on the Norweigan coast through various pipelines. The gas travels at an astonishing speed, reaching its destination in just 84 seconds, which means it passes through the pipes at almost 3,000 km/h (1,864 mph). 

Initially, Troll A used two compressors, which are used to extract the gas and compress it, forcing it at high speed through the pipeline. Two further compressors were later added to Troll A in 2014 because the gas pressure had dropped since operations began in 1996. To support these added compressors, a new module, containing living quarters and a new control room, was added in 2010. These additions meant higher energy consumption and new power cables were laid which connected Troll A with the mainland. However, because of grid constraints, should there be a grid emergency on the mainland, Troll A would effectively be cut off. 

Like its two other Troll cousins, Troll A uses horizontal wells, meaning two separate drilling operations. The first drill travels vertically 1,600 metres until it reaches the gas reservoir below the surface. The second moves down to 3,200 meters but in a slightly more horizontal direction. In 2008, the gas being pumped out across the three Troll platforms was roughly 120 million cubic metres (4.2 billion cubic ft) per day. Unfortauenly we don’t have specific data for exactly how much is currently being pumped out, but we do know that the Norwegian government has set a cap of 36 billion cubic metres (1.2 trillion cubic ft) per year.  


The platform cost $650 million (just over $1 billion today) to build and is currently operated by EQUINOR, a Norweigan state-run multinational energy company. 

The Troll A gas platform is astonishing for so many reasons. Obviously, its accolades of being the tallest and heaviest object ever moved should be enough, but the engineering that went into it is equally astonishing. The way that the giant legs were built and the process of manoeuvring the two sections together safely is just mind-boggling.

This is an extraordinary megaproject that seems to have flown well under the radar – which is ironic for its massive bulk. If Troll A stood on land, it would be the 13th tallest structure in the world but instead, its long legs reach deep into the cold depths of the North Sea, its vacuum anchors holding it firmly in place with frigid waves battering it day in day out. And there, the largest object ever moved shall sit for the next 46 years – at least.   

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